Source: Mrs Beeton's Book of Household Management (1861)
Bisque relies on extracting umami and body from prawn shells, then binding the liquid with a fine puree of the meat and bread. This is a stock-based approach: the shells and aromatics steep briefly in vinegar and water to draw out flavour without the long simmer that would cloud the broth, then that infusion marries with your main stock base. The vinegar's acidity denatures the protein in the shells, releasing glutamates faster than water alone would.
Shell the prawns, keeping the tails whole for garnish. Tip the bodies into a stewpan with the mace blade, vinegar, and 300 ml water. Stew for 15 minutes at a gentle simmer—vigorous heat will make the liquid greasy and bitter. Strain through muslin or a fine sieve, pressing gently on the shells to extract liquid without forcing through grit. Pour your main stock (fish or water) into a clean pan and add this infusion. Bring to a simmer.
Pound the reserved prawn tails with the moistened breadcrumbs into a fine paste—the bread acts as a binder, its starches and gluten holding the delicate prawn protein together. Work it through a fine sieve into the simmering soup in small batches, stirring constantly to avoid lumps. This emulsified seafood paste thickens the soup and deepens its colour to a pale blush-pink. The texture should be velvety, not grainy. Simmer for 10 minutes once all the paste is incorporated.
Season with anchovy sauce or mushroom ketchup (both are high in flavouring umami, so start cautiously), then balance with lemon juice to lift the richness. Taste as you go. If you need more body, whisk together cold butter and a small amount of flour (a beurre manié), crumble it into the soup off the heat, then whisk over low heat until it thickens without breaking. Add the whole reserved prawn tails and warm them through—no more than two minutes, or they toughen. Serve in hot bowls with a prawn tail floating on top.
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